The technology of electrophotography is commercially well established A wide variety o& processes and apparatus are used, although they have many characteristics in common. One of the more common forms of this technology involves the use of a plate having a photoconductive insulating layer, generally coated on a conductive layer. Imaging is effected by first uniformly electrostatically charging the surface of the photoconductive layer and then exposing the charged layer to an image or pattern of activating electromagnetic radiation, usually visible or ultraviolet radiation. This exposure selectively enables the charge in the irradiated areas of the photoconductive insulator to dissipate. The charge which remains in the non-irradiated areas forms a latent image which may be further processed to form a more permanent record of exposing image or pattern. The most common form of additional processing involves the attraction of particles of material electively to the charged or uncharged areas and fusing them to the photoconductive layer or transferring the particles in their imagewise distribution to another surface to which they are more permanently bound by an adhesive or by fusion of the particles themselves. A common electrophotographic construction comprises, in sequence, a substrate, a conductive layer, and a photoconductive insulating layer.
Typical classes of photoconductive materials useful in electrophotography include 1) inorganic crystalline photoconductors such as cadmium sulphide, cadmium sulphoselenide, cadmium selenide, zinc sulphide, zinc oxide, and mixtures thereof, 2) inorganic photoconductive glasses such as amorphous selenium, selenium alloys, and selenium-arsenic, and 3) organic photoconductors such as phthalacyanine pigments and polyvinyl carbazole, with or without binders and additives which extend their range of spectral sensitivity. These systems are well known in the art. For example, U.S. Pat. No. 3,877,935 discusses various problems associated with the crystalline and amorphous classes of photoconductors and shows the use of polynuclear quinone pigments in a binder as a photoconductive layer. U.S. Pat. No. 3,824,099 shows the use of squaric acid methine sensitizing dyes and triaryl pyrazoline charge transport materials as an electrophotographic construction. The use of poly-N-vinylcarbazole as a photoconductive insulating layers is disclosed in U.S. Pat. No. 3,037,861. A number of diverse organic photoconductors have been disclosed since the development of the carbazole class of photoconductors such as quinones and anthrone [e.g., Hayashi et al., Bull. Chem. Soc. Japan. vol 39, (1966) pp. 1670-73], but the carbazoles have continued to attract the greatest attention. Other carbazole photoconductors are disclosed in U.S. Pat. No. 4,025,341, Xerox Disclosure Journal, Vol 3, No. 1, Jan/Feb 1978, page 7, Japanese Patent Publication No. 52-34735 and European Patent Publication No. 58840.
Organic photoconductors have been spectrally sensitised with a wider range of sensitising dyes such as triarylmethane dyes, disulphone dyes, quinazoline dyes, polyquinoid dyes, polyanthraquinoic dyes, pyrylium-based dyes and cyanine dyes. Examples of the use of indocyanine derivatives as sensitising dyes are disclosed in U.S. Pat. Nos. 4,617,247 and 4,362,800.
Double layer organic photoconductor systems comprising separated but contiguous charge carrier generating (CGL) and charge carrier transporting layers (CTL) are well known. In such function separated constructions, the CGL absorbs incident light and generates charge carriers with subsequent injection of appropriate polarity carriers into the CTL wherein they migrate and neutralise charges of opposite polarity at the surface. Since all viable charge transport materials are hole transporting, conventional bi-layer organic photoconductors in which the CTL overlies the CGL are sensitive only to negative charging Negative corona charging is less stable in discharge as compared to positive charging and, additionally, produces large amounts of ozone which is not only environmentally undesirable but can also cause chemical changes on the organic photoconductor surface. Inverted structures, in which CGL overlies the CTL, would be sensitive to positive charging; but the practical problems involved in protecting the thin CGL from damage during storage and use makes the fabrication of such designs extremely difficult.
Single layer organic photoconductor materials in which a sensitising dye is uniformly dissolved or dispersed within the CTL matrix are sensitive to positive charging; but these have been found to be suitable only for single use or very limited re-use owing to their relatively poor maintenance of stable electrophotographic responses during recycling.
It has been found that the association e.g. dimerisation, of sensitising dyes in organic photoconductor coatings can result in the formation of charge carrier traps which have a detrimental effect on recycling of the photoconductor for reuse in imaging procedures and adversely affect photoresponse properties. In particular it has been observed that both charge acceptance decreases and that photosensitivity increases following a charge and discharge cycle. The time required for re-establishment of the initial charged state varies from minutes to many hours depending upon the particular dye used and its concentration.
It has now been found that a particular class of sensitising dye has little propensity for association in organic photoconductors, thereby greatly improving the stability of the photoreceptor response characteristics during electrophotographic recycling